A curable composition which is curable upon exposure to actinic rays such as ultraviolet rays or an electron beams, or heat has been utilized in various uses for paints for plastics, paper, woodwork and inorganic materials, adhesives, printing inks, printed circuit boards and electric insulators. Recently, further improvement in weather resistance and adhesiveness has been desired, specifically in printing inks, paints and adhesives. As an ink jet ink employing these, there are known ultraviolet ray curable ink jet inks. Also recently, an ink jet system employing these ultraviolet ray curable inks has been noted in terms of relatively low odor, high-speed drying and being recordable onto a recording medium having no ink absorptivity. There is disclosed ultraviolet ray curable ink jet ink, for example, in JP-A No. 6-200204 (hereinafter, the term JP-A refers to Japanese Patent Application Publication) and published Japanese translation-of PCT international publication for patent application No. 2000-504778. There has been desired, in this field, formation of a strong and soft film of superior adhesion as well as low viscosity.
Further, when using these inks, curing sensitivity is relatively easily variable, depending on the kind of recording material and working environment.
An ink using a radical polymerizable compound is subject to an oxygen-inhibiting action, tending to cause curing inhibition at a relatively low ink droplet amount. Contrary to that, an ink using a cation-polymerizable compound is not subject to such oxygen inhibition, in fact exhibiting no curing inhibition even at a relatively low droplet amount and then enabling printing of precise images. However, inks employing cation-polymerizable compounds, as described in JP-A Nos. 2001-220526, 2002-188025, 2002-317139 and 2003-55449 have problems of being easily affected by moisture (humidity) at the molecular level.